Process of heat treating steel



J y 6, 1938. E. u... ROBINSON PROCESS OF HEAT TREATING STEEL Filed Oct. 30, 1936 6 a f /apsed 7/076 in M/m/fes INVENTOR lTE Patented July 26, 1938 p a zaizs raoonss or near 'rnrnrme sonar.

Application Cctober so, 1936, Serial No. mates 5 trains. (Cl. ltd-421.5)

This invention relates to a process of heat treating steel. 4

In the fabrication of strip steel for deep drawing into shapes for'use in the difierent industries, the steel is subjected to hot and. cold rolling processes to reduce it to the desired thickness. This working of the steeLparticularly low carbon steel v containing up to .1% carbon, renders the steel hard and comparatively non-ductile.

After the steel is worked, it is usually subjected to a treatment known as the Bell annealing process which requires a time of approximately 48 hours. In practice, it has been found that heat treating the worked steel by the Bell annealing process is ineficient because of the amount of time necessary to render the steel ductile.

A short-cycle continuous-annealing. process such as that disclosed in copending application Serial No. 96,690, filed August 18, 1936, has also been employed to render the steel ductile, and

while the desired ductility is obtainedjthe resulting steel is sometimes so hard as to cause excessive die wear, buckling and spring back in the metal. It is, therefore, desirable to provide a process for heat-treating the worked low carbon steel whereby it will have a low hardness value and will be rendered ductile in a very short time.

An object of this invention is to provide for heat-treating low carbon steel to render it ductile.

Another object of this invention is to provide 0 a process for heat-treating steel for improving the hardness characteristic and ductility value of the steel. A

Other objects of this invention will become apparent from the following description'when taken in conjunction with the accompanying gas capable of preventing oxidation of the steel during the heat treatment such as hydrogen, dissociated ammonia, nitrogen, or a mixture of gases such as carbon monoxide, nitrogen, hydrogen and methane.

In order to release the strain set up by working the steel, it is heated to and maintained at a sufliciently high temperature in the protective atmosphere to effect the alpha to gamma iron transformation of the steel. This temperature may be between 935 C. and 1000 C. The heating to, and at, this temperature may be for a is quickly cooled to a temperature of between 900 C. and 700 C. to cause precipitation of the alpha iron. When the temperature of the steel is maintained between 900 C. and 700 C. for a period of time of about one minute, it is noted that there is a substantial completion of the precipitation of the alpha iron.

In order to cause a re-solution of the alpha iron and to obtain a more homogeneous solution of carbon and gamma iron, the steel is quickly heated from. the temperature of between 900 C. and 700 C. to a temperature of between 935 C.

and 1000" C. to again efiect the alpha to gamma iron transformation. The steel is maintained at the temperature of between 935 C. and 1000 C. for a period of time of about one minute. This short period is sufficient to obtain the desired re-solution of the alpha iron and carbides which may be present.

With the steel in this condition, it is quickly cooled to a temperature of about 700 C. andmaintained at that temperature for a period of time of between 3 and 4 minutes to cause a precipitation from the gamma solution of alphairon containing a maximum possible amount of dissolved carbon. In order to retain the dissolved carbon in solution in the alpha iron, the steel is then quickly cooled to a temperature of about 300 C. after which it is quickly heated to a temperature of about 630 C. to cause a precipitation of carbides from the alpha iron in inter and intra-crystalline formation. The steel may then be quickly cooled to room temperature with the carbides in the desired formation such as a globular form as opposed to the pearlitic form. Where desired, the'cooling of the steel from about 330C. to room temperature may include a pause at about 400 C. in order to allow completion of the carbide precipitation from the alpha iron 9.5;.-- 7

far as possible.

By subjecting the steel to the second heating at the temperatures between 935 C. and l000 C., it is found that the resulting steel has a larger grain size than where the steel is subjected to only one treatment at the high temperature. Although it is not known exactly what effect the larger grain size has on the results obtained, it has been noted in many experiments, that with the larger grain size the desired ductility may be obtained, together ,with low hardness values. Where low carbon steel has been worked to a thickness of .040 inch and then subjected to the heat treatment hereinbefore set forth, the resulting products have been found to have Olsen values ranging between 415 and 430, together with Rockwell hardness values of below 45.

In a particular modification of this invention, as shown by curve it of Fig. l of the drawing in which the ordinate represents temperature in degrees centigrade and the abscissa represents elapsed time 'in minutes, the heat-treating cycle comprises heating the steel in hydrogen from room temperature to about 950 C. in a period of time of about 1.75 minutes and maintaining the steel at that temperature for a period of time of about one minute to -a homogeneous solution of carbon in gamma iron. At the end of one minute the steel is quickly cooled in a period of time of about .75 minute to a temperature of about 700 C. and maintained at that temperature for a period of time of about one minute to cause and efiect the completion of a precipitation of alpha iron.

In order to cause a ire-solution of the alpha iron and to obtain a larger grain size in the steel,

the steel is then reheated to a temperature of about 950 C. and maintained at that temperature for a period of time of about one minute to insure a substantially homogeneous solution of carbon in the gamma iron. With the carbon thus in solution, the steel is then quickly cooled in a period of time of about .5 minute to a temperature of about 700 C. and held there for a period of time of about 3.5 minutes to effect a precipitation of alpha iron containing a maximum possible amount of dissolved carbon. In order to effect the retention of the solution to a large extent, the steel is then cooled from the temperature of about 700 C. to a temperature of about 300 C. in a period of time'of about 2.15 minutes, and

then immediately re-heated toa temperature of about 630 C. to cause a precipitation of dissolved carbides from solution in the alpha iron to give a mixture of the carbides and grains of alpha iron. With this precipitation of the carbides, the steel is then cooled in a period of time of about .5 minute to a temperature of about 425 C. and then slowly cooled during a period of time of about two minutes to a temperature of about 400 C. which permits completion of the ess of great commercial value. Although it is recognized that this process may occasionally araama produce a product havinga larger grain size than that obtained by the Bell annealing process and other similar processes, the usual increase in the grain size is not so excessive as to destroy the usefulness of the sheet.

Although this invention has been described with reference to a particular process, it is, of course, not to be limited thereto except insofar as is necessitated by the prior art and the scope of the appended claims.

' I claim as my invention:

1. A process of heat treating low carbon steel which comprises heating the steel in a protective atmosphere at a temperature above the alphagamma transformation temperature of the steel to obtain a solution of carbon in gamma iron, cooling the steel to a temperature of between 900 C. and 700 C. to cause a precipitation of alpha iron, holding the steel at the temperature of between 900 C. and 700 C. for a period of time sufllcientto permit the completion of the precipitation of alpha iron, subjecting the steel to a temperature above the alpha -gamma transformation temperature of the steel to obtain a'substantially homogeneous resolution of carbon in gamma iron, cooling the steel to a. temperature of about 300 C. to efiect a retention'of the carbon in solution, increasing the temperature of the steel to about 630 C. to cause a precipitation of carbides in the alpha iron, and quickly cooling the steel to render it ductile.

2. A process of heat treating low carbon s eel which comprises heating the steel in a protective atmosphere at a temperature above the alphagamma transformation temperature of the steel to obtain a solution of carbonin gamma iron, cooling the steel to a temperature of between 900 C. and 700 C. to cause a precipitation of alpha iron, holding the steel at the temperature of between 900 C. and 700 C. for a period of time sumcient to permit the completion of the precipitation of alpha iron, subjecting the steel to 'a temperature above the alpha-gamma transformation temperature of the steel to obtain a substantially homogeneous resolution of carbon in gamma iron, cooling the steel to a temperature of about 700 C. and maintaining it at that temperature for a period of time sufficient to cause a precipitation of alpha iron which will. contain a substantial amount'of dissolved carbon, cooling the steel to a. temperature of about 300 C. to effect a retention of the carbon in solution, increasing the temperature of the steel to about 630 C. to cause a precipitation of carbides in the alpha iron, and quickly'cooling the steel to render it ductile.

3. A process of heat treating low carbon steel which comprises heating the steel in a protective atmosphere at a temperature of above the alphagamma transformation temperature of the steel to obtaina solution of carbon in gamma iron, cooling the steel to a temperature of about 700 Cato cause a precipitation of alpha iron, holding the steel at the temperature of about 700 C. for a period of time sufllcient to permit the completion of the precipitation of alpha iron, subjecting the steel to a temperature above the alpha-gamma transformation temperature of the steel to obtain a substantially homogeneous resolution of carbon in gamma iron, cooling the steel to a temperature of about 700 C. and maintaining it at that temperature for a period of time sufllcient to cause a precipitation of alpha iron which will contain a substantial amount of dissolved carbon, cooling the steel to a temperature of about 300 C. to effeet a retention of the carbon in solution, increasaromas ing the temperature of the steel to about 630 C. to causea precipitation of carbides in the alpha iron, cooling the steel to a temperature of beatmosphere at a temperature above the alphagamma transformation temperature of the steel to obtain a solution carbon in gamma iron, cooling the steel to a temperature of about 700 C. to cause a precipitation of alpha iron, holding the steel at the temperature of about 700 C. for a period of time of about one minute to permit the completion of the precipitation of alpha iron, subjecting the steel to a temperature above the alpha-gamma transformation temperature of the steel to obtain a substantially homogeneous resolution of carbon in gamma iron, cooling the steel to a temperature or about 700 C. and maintaining it at that temperature for a period of time of about 3.5 minutes to cause a precipitation of alpha iron which will contain a substantial amount of dissolved carbon, quickly cooling the steel to a temperature of about 300 C. to eilect a retention of the carbon in solution, increasing the temperature of the steel to about 630 C. to cause a precipitation of. carbides in the alpha iron, cooling the steel to a temperature of about 400 C. and'maintainina' it at that temperature for about 2 minutes to obtain a substantial completion of the precipitation of carbides, and quickly cooling the steel to render it ductile.

5. A process of heat treating low carbon steel which comprises heating the steel in a protective atmosphere to a temperature of about 950 C., which is above the alpha-gamma transformation temperature of the steel, and maintaining that temperature for a period of time of about one minute to obtain a solution of carbon in gamma iron, quickly cooling the steel to a temperature of about 700 C. to cause a precipitation of alpha iron, holding the steel at that temperature for a period of time of about one minute topermit the completion of the precipitation of alpha iron, heating the steel to a temperature of about 950 C. and maintaining that temperature for a period, ai time of about one mln'ute to obtain a substantially homogeneous resolution of carbon in gamma iron, quickly cooling the steel to a temperature of about 700 C. and maintaining that temperature for a period of time of about 3.5 minutes to cause a precipitation 0! alpha iron which will contain a substantial amount of dissolved carbon, quickly cooling the steel to a temperature of about 300 C. to eifect a retention of the carbon in solution, heating the steel to a temperature of about 630 C. to cause a precipitation oi carbides in the alpha iron, quickly cooling the 1 steel to a temperature of about 400 C. and mainto render it ductile.

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